arXiv subjects

Carlos Viscasillas Vázquez

Publications and source records attributed to Carlos Viscasillas Vázquez.

8 recordsLinked to original sources

arXiv2026

The oldest low-$α$ thin disc stars

The formation pathway of the $α$-poor Milky Way thin disc remains under debate. The latest observational data indicates the presence of comparable numbers of old stars in both the low- and high-$α$ disc components, suggesting either coeval or overlapping formation epochs. Using GALAH DR4, we provide evidence that the $α$-poor stars coexist with $α$-rich stars up to $\sim$12.5 Gyr. Stellar ages of 0.5 Gyr precision were re-derived for nearly $30\,000$ subgiant stars using accurate effective temperatures ($T_{\rm{eff}}$) from the infrared flux method (IRFM) together with distances based on Gaia parallaxes. About 22\% of the sample constitutes a population peaking at 9.2 Gyr, with evidence for an extended old-age tail reaching 12.5 Gyr. Among stars older than 10 and 11 Gyr, between 20 and 30\% have relative low [$α$/Fe] and solar-like metallicities ($-0.5 \leq \mathrm{[Fe/H]} \leq +0.5$ dex). These stars exhibit kinematics consistent with those of near solar-age $α$-poor stars in the thin disc. These stars can be evolved Sun-like stars, i.e. old solar analogues, whose $T_{\rm{eff}}$ and gravity modified with the time. Comparison with a well-tested Galactic disc chemical evolution model supports the presence of a significant fraction of old, $α$-poor disc stars, confirming the plausibility of our selection criteria. We provide a catalogue with the selected sample to facilitate spectroscopic follow-ups at high resolution.

astro-ph.SR↗

arXiv2026

Tracing the early Milky Way thin disc with the Gaia-ESO Survey

The origin of metal-poor stars on thin-disc-like orbits remains an open question in Galactic archaeology and provides important constraints on the earliest phases of Milky Way disc formation. We aim to identify and characterise metal-poor stars with thin-disc-like kinematics observed in the Gaia-ESO Survey, using Gaia DR3 orbital parameters, spectroscopic information, isochrone ages, and detailed chemical abundances. Out of 1784 turn-off stars, we identify one metal-poor candidate with [Fe/H] = -1.38 on a dynamically cold, prograde orbit, with low eccentricity, high azimuthal velocity ($V_ϕ$ approximately 220 km/s), large angular momentum and low vertical action. Chemically, however, it is clearly distinct from the canonical thin disc: it is $α$-enhanced, with [Mg/Fe] = +0.58 and [$α$/Fe] = +0.48, shows a low [Y/Mg] = -0.58 ratio, and lies on the metal-poor sequence in the [Al/Fe]-[Mg/Mn] plane. The star also has an old age estimate, with a most probable value of about 10.2 Gyr. Its properties suggest that it is not a typical member of the canonical thin disc. Instead, it may be associated with the old thin disc, the metal-weak thick disc, or prograde halo-related populations. This object provides a useful benchmark for studying early metal-poor populations on disc-like orbits.

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arXiv2025

Mapping the Ages of Stars with Chemistry

Chemical clocks, based on age-sensitive stellar abundance ratios, offer a powerful and scalable approach to reconstruct the formation history of the Milky Way. This white paper outlines how wide-field, high-resolution spectroscopy can transform chemical clocks into precise and broadly applicable stellar age estimators when combined with astrometry and asteroseismology. We summarize the current limitations, including calibration across Galactic environments and the impact of internal stellar evolution, and define the observational requirements needed to overcome them. The Wide-field Spectroscopic Telescope (WST), with its large field of view, high multiplex, and broad wavelength coverage at high spectral resolution, is uniquely suited to deliver the homogeneous datasets required to map the age structure of the Galaxy at unprecedented scale. Such a capability will enable decisive progress in Galactic archaeology and stellar evolution studies.

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arXiv2024

Modelling chemical clocks -- Theoretical evidences of the space and time evolution of [s/alpha] in the Galactic disc with Gaia-ESO survey

Chemical clocks based on [s-process elements/alpha-elements] ratios are widely used to estimate ages of Galactic stellar populations. However, the [s/alpha] vs. age relations are not universal, varying with metallicity, location in the Galactic disc, and specific s-process elements. Current Galactic chemical evolution models struggle to reproduce the observed [s/alpha] increase at young ages. We provide chemical evolution models for the Milky Way disc to identify the conditions required to reproduce the observed [s/H], [s/Fe], and [s/alpha] vs. age relations. We adopt a multi-zone chemical evolution model including state-of-the-art nucleosynthesis prescriptions for neutron-capture elements (AGB stars, rotating massive stars, neutron star mergers, magneto-driven supernovae). We explore variations in gas infall, AGB yield dependencies on progenitor stars, and rotational velocity distributions for massive stars. Results are compared with open cluster data from the Gaia-ESO survey. A three-infall scenario for disc formation captures the rise of [s/alpha] with age in the outer regions but fails in the inner ones, especially for second s-process peak elements. Ba production in the last 3 Gyr of chemical evolution would need to increase by half to match observations. S-process contributions from low-mass AGB stars improve predictions but require increases not supported by nucleosynthesis calculations, even with potential i-process contribution. Variations in the metallicity dependence of AGB yields show inconsistent effects across elements. Distributions of massive star rotational velocities fail to improve results due to balanced effects on elements. We confirm that there is no single relationship [s/alpha] vs. age, but that it varies along the MW disc. Current prescriptions for neutron-capture element yields cannot fully capture the complexity of evolution, particularly in the inner disc.

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arXiv2024

Mapping radial abundance gradients with Gaia-ESO open clusters: Evidence of recent gas accretion in the Milky Way disk

Context. Recent evidences from spectroscopic surveys point towards the presence of a metal-poor, young stellar population in the chemical thin disk. In this context, the investigation of the spatial distribution and time evolution of precise, unbiased abundances is fundamental to disentangle the scenarios of evolution of the Galaxy. Aims. We study the evolution of abundance gradients in the Milky Way by taking advantage of a large sample of open clusters, which are among the best tracers for this purpose. In particular, we use data from the last release of the Gaia-ESO survey. Methods. We perform careful selection of open cluster stars excluding those that may be affected by biases in spectral analysis. The cleaned open clusters sample is compared with detailed chemical evolution models for the Milky Way, using well tested stellar yields and prescription for radial migration. Different scenarios of Galaxy evolution are tested to explain the data, i.e. the two-infall and the three-infall frameworks, suggesting that the chemical thin disk is formed by one or two subsequent gas accretion episodes, respectively. Results. With the performed selection in cluster stars, we still find a metallicity decrease between intermediate age (1<Age/Gyr<3) and young (Age<1Gyr) open clusters. This decrease cannot be explained in the context of the two-infall scenario, even by accounting for the effect of migration and yield prescriptions. The three-infall model, with its late gas accretion in the last 3 Gyr, can explain the low metallic content in young clusters. However, we invoke a milder metal dilution relative to previous findings. Conclusions. To explain the observed low metallic content in young clusters, we propose that a late gas accretion episode triggering metal dilution should have taken place, extending the framework of the three-infall model for the first time to the entire Galactic disk.

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arXiv2023

The role of radial migration in open cluster and field star populations with Gaia dr3

The survival time of a star cluster depends on its total mass, density, and thus size, as well as on the environment in which it was born and in which lies. Its dynamical evolution is influenced by various factors such as gravitational effects of the Galactic bar, spiral structures, and molecular clouds. Overall, the factors that determine the longevity of a cluster are complex and not fully understood. This study aims to investigate if open clusters and field stars respond differently to the perturbations that cause radial migration. In particular, we aim at understanding the nature of the oldest surviving clusters. We compared the time evolution of the kinematic properties of two Gaia DR3 samples: the first sample is composed of $\sim$40 open clusters and the second one of $\sim$66,000 MSTO field stars. Both selected samples are composed of stars selected with the same quality criterion, belonging to the thin disc, in a similar metallicity range, located in the same Galactocentric region [7.5-9 kpc] and with ages >1 Gyr. We performed a statistical analysis comparing the properties of the samples of field stars and of open clusters. A qualitative comparison of kinematic and orbital properties reveals that clusters younger than 2-3 Gyr are more resistant to perturbations than field stars and they move along quasi-circular orbits. Conversely, clusters older than approximately 3 Gyr have more eccentric and inclined orbits than isolated stars in the same age range. Such orbits lead them to reach higher elevations on the Galactic plane, maximising their probability to survive several Gyr longer. A formal statistical analysis reveals that there are differences among the time evolution of most of the kinematic and orbital properties of field stars and open clusters. Our results suggest that oldest survived clusters are usually more massive and move on orbits with higher eccentricity.

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arXiv2023

Origin of neutron capture elements with the Gaia-ESO survey: the evolution of s- and r-process elements across the Milky Way

We study the abundance patterns and the radial gradients of s-process elements (Y, Zr, Ba, La and Ce), r-process elements (Eu) and mixed-process elements (Mo, Nd and Pr) in the Galactic thin disc by means of a detailed two-infall chemical evolution model for the Milky Way with state-of-the-art nucleosynthesis prescriptions. We consider r-process nucleosynthesis from merging neutron stars (MNS), magneto-rotational supernovae (MR-SNe) and s-process synthesis from low- and intermediate- mass stars (LIMS) and rotating massive stars. The predictions of our model are compared with data from the sixth data release of the Gaia-ESO survey, from which we consider 62 open clusters with age > 0.1 Gyr and 1300 Milky Way disc field stars. We conclude that: i) the [Eu/Fe] vs. [Fe/H] is reproduced by both a prompt and a delayed source, but the quick source completely dominates the Eu production; ii) rotation in massive stars contribute substantially to the s-process elements of the first peak, but MNS and MR-SNe are necessary in order to reproduce the observations; iii) due to the adopted yields, our model overpredicts Pr and underpredicts Nd, while the [Mo/Fe] vs. [Fe/H] is nicely reproduced. For the radial gradients, we conclude that: i) our predicted slope of the [Fe/H] gradient is in agreement with the one observed in open clusters by Gaia-ESO and other high-resolution spectroscopic surveys. ii) The predicted slope of the [Eu/H] radial gradient is steeper than the observed one, independently on how quick the production of Eu is. We discuss the possible causes of this discrepancy in terms of both different Galaxy formation scenarios and stellar radial migration effects. iii) For all the elements belonging to the second s-process peak (Ba, La, Ce) as well as for Pr, we predict a plateau at low Galactocentric distances, which is probably due to the enhanced enrichment from LIMS in the inner regions.

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arXiv2021

Abundances of neutron-capture elements in thin- and thick-disc stars in the solar neighbourhood

The aim of this work is to determine abundances of neutron-capture elements for thin- and thick-disc F, G, and K stars in several sky fields near the north ecliptic pole and to compare the results with the Galactic chemical evolution models, to explore elemental gradients according to stellar ages, mean galactocentric distances, and maximum heights above the Galactic plane. The observational data were obtained with the 1.65m telescope at the Moletai Astronomical Observatory and a fibre-fed high-resolution spectrograph. Elemental abundances were determined using a differential spectrum synthesis with the MARCS stellar model atmospheres and accounting for the hyperfine-structure effects. We determined abundances of Sr, Y, Zr, Ba, La, Ce, Pr, Nd, Sm, and Eu for 424 thin- and 82 thick-disc stars. The sample of thick-disc stars shows a clearly visible decrease in [Eu/Mg] with increasing [Fe/H] compared to the thin-disc stars, bringing more evidence of a different chemical evolution in these two Galactic components. Abundance correlation with age slopes for the investigated thin-disc stars are slightly negative for the majority of s-process dominated elements, while r-process dominated elements have positive correlations. Our sample of thin-disc stars with ages spanning from 0.1 to 9 Gyrs give the [Y/Mg]=0.022 ($\pm$0.015)-0.027 ($\pm$0.003)*age [Gyr] relation. For the thick-disc stars, when we also took data from other studies into account, we found that [Y/Mg] cannot serve as an age indicator. The radial [El/Fe] gradients in the thin disc are negligible for the s-process dominated elements and become positive for the r-process dominated elements. The vertical gradients are negative for the light s-process dominated elements and become positive for the r-process dominated elements. In the thick disc, the radial [El/Fe] slopes are negligible, and the vertical slopes are predominantly negative.

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